1. Field of the Invention
The present invention relates to an antenna, more particularly to a multi-band antenna, the entire disclosure of which is incorporated herein by reference.
2. Description of the Related Art
In recent years, more and more consumer electronic devices with communication functionality have been developed with the growing availability of various wireless communication frequency bands. Since different generations of communication systems are being introduced in every few years, smart phones and portable computers need to be compatible not only with older communication systems such as Second Generation Wireless Telephone Technology (2G) and 3rd Generation (3G) wireless telephone technology, but also with newer communication systems such as Long Term Evolution (LTE) systems. Therefore, it is desirable to have an electronic device capable of operating at various wireless communication frequency bands.
A conventional solution for the electronic device to be compatible with various frequency bands is to provide multiple antennas, e.g., one of the antennas is for 2 G communication system, and another one of the antennas is for 3 G communication system. However, more space is required in such electronic devices, thereby making it difficult to reduce the size of the electronic devices so as to comply with the current trend toward miniaturization. Consequently, it is desirable to have a single antenna capable of operating at various wireless communication frequency bands.
Referring to FIG. 1, U.S. Pat. No. 7,050,010 discloses a multi-band antenna compatible with dual-bands and having a return loss frequency response shown in FIG. 2. One of resonant frequency bands approximate to 2.4 GHz is composed of a resonant mode, and the other one of the resonant frequency bands approximate to 5 GHz is composed of two resonant modes. Although the above mentioned antenna is capable of operating at multiple frequency bands, the frequency band approximate to 2.4 GHz is composed of a single resonant mode and thus has a limited bandwidth. Hence, it is difficult to satisfy operating requirements for LIE system (13/17) and GSM850/GSM900 systems (704 MHz˜960 MHz) by simply adjusting the size of the antenna.
Referring to FIG. 3, Taiwanese Utility Model No. M391734 discloses a Long Term Evolution (LTE) antenna that is simultaneously compatible with LTE band 13, Global System for Mobile Communications (GSM), Digital Cellular System (DCS), Personal Communication System (PCS), and Wideband Code Division Multiple Access (WCDMA) communication systems and that has a return loss frequency response shown in FIG. 4. The LTE antenna comprises a circuit board 11, a monopole antenna 12, a coupling element 13 having first and second coupling portions 131, 132, and a capacitor 14. The first coupling portion 131, the monopole antenna 12, and the second coupling portion 132 overlap in a vertical direction in the drawing such that electromagnetic energy thereof couple with each other. Once the resonant mode covering a frequency band ranging from 1710 MHz˜2170 MHz is adjusted, the resonant mode covering frequency band ranging from 746 MHz˜946 MHz will be affected, thereby resulting in frequency offset and impedance mismatch, and increasing difficulty in designing the antenna. Additionally, use of the capacitor 14 is required in such antenna, which results in a cumbersome manufacturing process and increase of manufacturing cost.